This invention relates generally to sputtering systems, and more particularly, to magnetically enhanced sputtering apparatus having an elongated target arrangement.
The application of magnetic field enhancement to an elongated cathode configuration in a gas discharge sputtering device is described in U.S. Pat. No. 3,878,085, which issued to J. F. Corbani on Apr. 15, 1975. Corbani confirms the discovery reported in U.S. Pat. No. 2,146,025 of Penning that the power consumption of a sputtering system can be greatly reduced, and the quality and deposition rate of the coating on a substrate improved, by magnetically retaining the electrons of the gas discharge in a region near the cathode of the sputtering system. In the Corbani apparatus, retention is achieved by providing a tunnel-like magnetic field adjacent to the target surface in which the charged particles move.
When the tunnel-like magnetic field is made to close on itself to form a closed loop adjacent to the sputtering target surface, the sputtering process becomes very efficient because the electrons can no longer escape from the ends of the tunnel. Sputtering methods using the closed loop approach have been given the name "magnetron sputtering," and the surface of the cathode enclosed by the magnetic tunnel is called the "race-track."
Sputtered material emanates from a magnetron sputtering cathode predominantly from the race-track region. The distribution of this material is usually very non-uniform. Uniform film coating on a substrate is therefore accomplished by some means of moving the substrate, which has the effect of averaging and thereby smoothing the sputtered deposit. Thus, the practical applications of magnetron sputtering have resulted from the combination of a particular magnetic tunnel and target configuration (which defines the race-track) with an appropriate substrate motion to achieve a system providing acceptable coating uniformity. Magnetically enhanced sputtering has been the subject of extensive patent activity. (U.S. Pat. Nos. 2,146,025, 3,282,816, 3,884,793, 3,995,187, 4,030,996, 4,031,424, 4,041,353, 3,711,398, 4,060,470, 4,111,782, 4,116,793, 4,194,962, 4,166,018 and 4,198,283).
This invention relates to that type of system which combines a sputtering cathode, having an elongated closed loop magnetic tunnel which encloses a correspondingly-shaped sputtering surface (race-track), with a linear translation of the substrate past the sputtering surface.
The preferred practice regarding the use of such a cathode involves scanning the substrates past the cathode, usually in a plane parallel to the plane defined by the race-track and separated therefrom by a distance referred to as the target-to-substrate distance. Such motion is further usually directed perpendicular to the long axis of the race-track. By convention, that substrate dimension which lies in the plane of motion and extends transverse to the scanning direction is referred to as the substrate width, and that which extends parallel to the scanning direction is referred to as the substrate length.
There are two types of sputtered material loss which occur when using the above configuration. The first arises because coating uniformity requires that the long axis of the race-track exceed the width of the substrate. For example, to assure a coating thickness non-uniformity of no more than .+-.10% across the substrate width typically requires that the race-track length be 3" to 4" longer than this width; i.e., coating a 12" wide substrate requires a 15" to 16" long race-track region. Thus, sputtered material accumulates on apparatus components which lie opposite the target and adjacent to the width dimensions of the substrate. This constitutes a loss of sputtered material which is here referred to as target end loss.
The second type of material loss arises when a substrate of finite length is to be coated. Here, a common practice consists of positioning the substrate leading edge immediately adjacent to the deposition zone of the sputtering target. Sputtering is then commenced and scanning motion of the substrate initiated. This process is maintained until the trailing edge of the substrate clears the deposition zone. If the deposition zone of the cathode has a lenght l, measured in the scan direction, and the substrate has a length L, then the total distance traversed by the substrate is the distance l+L. However, during part of this process, the substrate does not cover the deposition zone, so sputtered material deposits onto parts of the sputtering machine in front of or behind the substrate. The fraction of material thus lost is given by the ratio l/(l+L). This loss (called an overscan loss) can be minimized by reducing l, the length of the film deposition zone produced by the cathode.
Both overscan losses and target end losses cause sputtered material accumulation on apparatus components. This requires frequent process interruption for removing the unwanted material and subsequent cleaning of the apparatus components.
The principal object of this invention is to provide a cathode which focuses the sputtered material emanating from an elongated race-track region to reduce the length of the deposition zone, thereby reducing overscan losses, and also to reduce the width of the deposition zone, thereby minimizing target end losses. In addition, since focusing decreases the area over which deposition occurs, the deposition rate in the smaller resulting deposition zone of the target is correspondingly increased.
It is known that the incorporation of residual gas contaminants in the coating is controlled by the mathematical relationship R/P, where R is the film deposition rate, and P is the residual gas pressure. Thus, an increase in deposition rate R which results from the above focusing permits an increase in the allowable residual gas pressure P without exceeding acceptable limits of coating impurity. The increase in allowable residual gas pressure P permits the use of lower quality vacuum pumping equipment and shorter pump-down times prior to starting deposition, and it generally makes the coating process more tolerant to small air leaks in the sputtering system. This advantage is especially important in the deposition of aluminum and its alloys in the manufacture of semiconductor integrated circuits, which deposition is adversely affected by sputtering system residual gas contamination.
A further advantage of such focusing is that it reduces waste of sputtering material, by reducing the amount of material deposited onto the structural components of the sputtering machine, and reduces system down-time for cleaning and vacuum pump-down time prior to operation of the system.